1. The Field of the Invention
This application relates to optical transmitters and, more particularly, to optical transmitters incorporating a directly modulated laser.
2. The Relevant Technology
U.S. patent application Ser. No. 11/272,100, filed Nov. 8, 2005 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM discloses a laser transmitter demonstrating error free transmission of 10 Gb/s signal at 1550 nm in standard single mode fiber with distance longer than 200 km without dispersion compensation using a directly modulated laser coupled to a passive optical spectrum reshaper. One element in such transmitters, and other frequency shift keying technology, is a laser source with substantially flat frequency modulation response from low frequencies up to the frequency comparable to the bit rate of the transmission systems, e.g., 1 MHz to 10 GHz for a 10 Gb/s digital signal.
Distributed feedback (DFB) lasers may be used to achieve the desired flat frequency modulation. Both fixed wavelength application and small-range wavelength tunable application based on temperature tuning (˜0.1 nm/° C.) have been demonstrated. One way to get large range wavelength tunable, and flat frequency modulation is to perform gain modulation of a distributed Bragg reflector (DBR) laser.
Referring to FIG. 1, a DBR laser 10 generally consists of a wavelength tuning section 12 and a gain section 14. The wavelength tuning section 12 may contain a DBR section 16 and a phase section 18. The DBR section 16 may serve as the coarse wavelength tuning section whereas phase section 18 is the fine wavelength tuning section. Both DBR section 16 and phase section 18 are preferably a same-material system with a photo luminescence wavelength (for example, 1.3 μm to 1.45 μm) below the lasing wavelength (for example, 1.55 μm) in order to avoid excessive loss. By injecting current into the DBR section 16 and the phase section 18, the carrier density in these sections will change. Due to the plasma effect, the refractive index of these sections will change, resulting in both peak reflection wavelength change and cavity mode frequency shift in the DBR section 16 and a cavity mode frequency shift in the phase section 18. By individual control of these two sections 16, 18, continuous wavelength tuning with high side mode suppression ratio has been demonstrated. Various configurations of DBR lasers with full C-band tuning capability have been used as tunable continuous wave (CW) sources recently.
To generate high speed frequency modulation, the gain section 14 of the DBR laser 12 is modulated with current modulation using the same principle as the gain modulated DFB laser. When the gain current of the DBR laser 12 is modulated, the photon density in the cavity is modulated. This in turn modulates the carrier density of the gain section 14 due to gain compression. The change of the carrier density results in a change in the refractive index of the gain section 14 and therefore changes the frequency of the laser 12. The frequency modulation of the laser 12 is sometimes referred to as chirp.
The frequency modulation properties of lasers have been studied significantly, and generally includes two parts: the carrier density effect and the thermal effect. (Diode lasers and photonic integrated circuits, Larry A. Coldren, Scott W. Corzine, Wiley interscience, page 211 to 213).
A typical small signal modulation frequency modulation response typically includes the carrier effect, including transient chirp, adiabatic chirp, and thermal effect. The thermal chirp generally has a time constant of ˜20 ns. Methods for suppressing thermal chirp have been demonstrated.
The FM small signal response induced by the gain current can be expressed according to the following equations:
                                          Δ            ⁢                                                  ⁢            v                                Δ            ⁢                                                  ⁢                          I              g                                      =                                            (                                                Δ                  ⁢                                                                          ⁢                  v                                                  Δ                  ⁢                                                                          ⁢                                      I                    g                                                              )                        Carrier                    +                                    (                                                Δ                  ⁢                                                                          ⁢                  v                                                  Δ                  ⁢                                                                          ⁢                                      I                    g                                                              )                        thermal                                              (                  Eq          .                                          ⁢          1                )                                                      (                                          Δ                ⁢                                                                  ⁢                v                                            Δ                ⁢                                                                  ⁢                                  I                  g                                                      )                    Carrier                =                                            v              adiabatic                        ⁡                          (                              1                +                                  j                  ⁢                                                                          ⁢                                      ω                    /                                          γ                      PP                                                                                  )                                ⁢                                    ω              R              2                                                      ω                R                2                            -                              ω                2                            +                              j                ⁢                                                                  ⁢                γ                ⁢                                                                  ⁢                ω                                                                        (                  Eq          .                                          ⁢          2                )                                                      (                                          Δ                ⁢                                                                  ⁢                v                                            Δ                ⁢                                                                  ⁢                                  I                  g                                                      )                    Thermal                =                              -                          v              thermal                                            (                          1              +                              j                ⁢                                                                  ⁢                ω                ⁢                                                                  ⁢                                  τ                  thermal                                                      )                                              (                  Eq          .                                          ⁢          3                )            
In the above three equations: νadiabatic represents the adiabatic chirp efficiency (typically ˜0.2 GHz/mA), νthermal represents the thermal chirp efficiency (typically ˜0.2 GHz/mA), γPP is ˜30 GHz, γ is ˜30 GHz, fr: is ˜10 GHz, and τthermal is ˜20 ns. (Diode lasers and photonic integrated circuits, Larry A. Coldren, Scott W. Corzine, Wiley interscience, page 211 to 213).
When the gain section 14 of the DBR laser 12 is modulated, another type of non-flatness of frequency modulation is identified, the frequency modulation small signal response is shown in FIG. 2. As is apparent from FIG. 2, segment 20 of the frequency response curve illustrates diminished response to frequency modulation. The non-flatness of the small signal response in the frequency range of 10 to 100 MHz can also be seen from time domain FM analysis with a “1010” data pattern at data rate of 100 Mb/s as shown in FIG. 3. Segments 22a, 22b of the time domain response illustrate a delayed chirp response on the rising and falling edges of the laser output in response to a square wave signal.
In the small signal response domain, slow chirp may be defined as the non-flatness of the FM small signal response between 10 to 100 MHz. The high frequency (>=10 GHz) non-flatness results from transient chirp and laser intrinsic speed, the low frequency (<10 MHz) non-flatness results from thermal chirp. In the time domain, slow chirp is defined as changes in frequency modulation that do not match the profile of the modulating current.
When modulated with high data rate, e.g. 10 Gb/s, this slow chirp will result in pattern dependence that deteriorates the transmission signal. It therefore would be an advancement in the art to improve the FM efficiency of DBR lasers by reducing slow chirp.